Several methods have been employed in the manufacture of silica tubing, e.g., by melting silica sand in a crucible and drawing off molten silica tubes which are subsequently solidified in a non-oxidizing atmosphere. Such method, though in wide commercial use, produces silica tubing having numerous trapped bubbles therein, known as air lines or striations. Such striations are highly undesirable in many applications of silica tubing, e.g., in lamp manufacture since the striations cause ridges, optical distortions, losses of strength and difficulties in sealing of tube ends which can be the cause of significant rejection rates and economic losses.
It has previously been believed that the main cause of bubbles in the molten silica resulted from gas entrapment in the void spaces of particulate or crystalline refractory materials, e.g., silica sand or quartz crystals which when poured on top of the molten silica mass in the melter caused the gases of the atmosphere in the melter to be trapped in pockets between such materials and carried into the melt as bubbles. Subsequent drawings of the melt into silica tubing then elongated the bubbles into striations as discussed above.
Commercial efforts to solve the above striation problems are well documented. For example, U.S. Pat. No. 3,717,450 discloses a process for the manufacture of quartz tubing having reduced striations therein in which filled, evacuated, fused silica tubes are successively lowered into a silica melt in a furnace with a need to continually replace the so-filled tubes. The furnace is surrounded in an atmosphere of non-oxidizing gas, e.g., 90% nitrogen -10% hydrogen. Such process, while successful, is partially intermittent and requires replacement of a succession of sand-filled quartz tubes as aforesaid.
In another attempt to reduce the striations in drawn fused silica tubing, U.S. Pat. No. 3,764,286 discloses feeding silica sand to the upper portion of a heated crucible onto a molten mass of silica in an atmosphere of 40-65% hydrogen and 60-35% helium, the crucible temperature being maintained above 2050.degree. C. The crucible is surrounded by an atmosphere of hydrogen in at least 80% by volume of nitrogen. The fused silica is drawn from a lower zone of the crucible through such atmosphere to obtain the silica tube product. Such method, however, requires a relatively high percentage of hydrogen in the crucible atmosphere and further requires a tungsten-lined crucible which is expensive compared to molybdenum as a crucible material.
Accordingly, the methods of the prior art have been equipment-oriented, expensive attempts to solve the above striation problem, and there is a need and market for continuously forming a fused silica product having reduced striations therein that is economical, uncumbersome and obviates the above prior art shortcomings.
There has now been discovered a method for continuously or intermittently forming a fused silica product that is markedly reduced in striations, which method is a departure from the attempts of the prior art to reduce striations in fused silica products, which present method lends itself to mass production at reduced cost.